This invention relates to a method and apparatus for converting analog output signals of a plurality of light receiving elements forming a photoelectric converting device to digital signals.
One of such analog-digital converting methods and apparatuses has been known from U.S. Pat. No. 4,255,654. In this known method and apparatus output photoelectric signals of a number of light receiving elements are simultaneously sampled and held by means of sample and hold circuits each connected to respective light receiving elements. The photoelectric signals thus held are supplied to one inputs of a number of comparators, respectively, to the other inputs of which is commonly supplied an analog reference signal whose amplitude varies successively in step-shaped manner. A number of digital memories are provided, to which memories a digital signal whose value varies successively according to the analog reference signal is applied in parallel and each of which memories is controlled by means of one of the comparators to store an instantaneous value of the digital signal which is supplied thereto just when the corresponding comparator changes its state. The digital signals stored in the memories are then read out therefrom as analog-digital converted output signals under the control of a central controller.
According to this analog-digital converting method and apparatus, all of the analog photoelectric signals held in the sample and hold circuits are converted into digital signals in parallel within a short time interval during which the analog reference signal varies from its lower limit value to its upper limit value, so that a number of analog photoelectric signals can be converted to digital signals at a very high speed. Accordingly, if this converting method is used for detecting a focus condition of a camera etc., the focus condition can be detected correctly for a fast moving object.
However, in the above mentioned analog-digital converting apparatus and method an analog-digital conversion range is determined by the analog reference signal varying in stepwise manner whose lower and upper limit values are fixed. Therefore if the output signals of the light receiving elements whose dark current or dark voltage increases with an increase of ambient temperature are converted to digital signals, a dynamic range of the analog-digital conversion is reduced with the increase of ambient temperature, because a dynamic range of the output signals of the light receiving elements is reduced with the increase of ambient temperature so that a lower step portion of the analog reference signal does not contribute to the analog-digital conversion.
FIG. 1 illustrates the relation between the dynamic range of the analog-digital conversion and the dark current component of the light receiving element which component increases with temperature increase. In the prior method and apparatus the analog-digital conversion of the output signals of the light receiving elements is effected within a range between the fixed lower and upper limit values of the analog reference signal. Therefore, it can be seen from FIG. 1 that if the dark current component of the light receiving element increases with temperature increase, an effective portion or range of the reference signal which contributes to the analog-digital conversion is reduced. As a result the dynamic range of the analog-digital conversion is reduced with temperature increase and thus the accuracy of the analog-digital conversion is decreased.